Color television systems encode "luminance" (brightness) and "chrominance" (hue) information signals in a composite television signal. The luminance signal is equal to a sum of weighted intensity values of the primary hues (red, blue, green). A pair of chrominance signals contain color information consisting of the corresponding differences between the intensities of the red and blue and the luminance signal. In the SECAM ("sequential color and memory") standard, the composite signal containing the luminance and chrominance information is provided in frequency modulated form. The composite SECAM signal comprises a luminance signal in one frequency band and sequentially alternating, frequency modulated chrominance signals in an adjacent, partially overlapping frequency band. The luminance spectrum extends from 0 to 6 MHz. The chrominance information modulates a subcarrier having a frequency swing that extends from 3.9 to 4.75 MHz. Interference between the luminance signal and the chrominance signals, a phenomenon known as "color crosstalk", can occur due to the overlapping frequency bands. This color crosstalk can cause receiver error characterized by a scrambled video image at the receiver.
The SECAM standard specifies attenuation of the amplitude of luminance components located in the overlapping part of the luminance spectrum in order to reduce color crosstalk. Such filtering is specified to have a nonlinear response that depends on the amplitude of the luminance signal in the chrominance bandwidth. Known analog notch filters use a classical passive filter with a set of diodes providing the nonlinear response. Another analog approach uses amplitude thresholding in response to repetitive high luminance signal levels to operate an anti-crosstalk trap circuit.